Spermatogenesis in rats is characterized by the synthesis of unique nuclear proteins at specific stages of cell differentiation. During spermiogenesis, the period of spermatid maturation, the histones and the acidic nonhistones are replaced by nuclear proteins which are more basic in amino acid composition, and the two major spermatidal proteins involved in this transition are designated TP and TP2. This histone to spermatidal basic protein transition is accompanied by nuclear elongation and chromatin condensation. We propose to isolate and further characterize TP2. As an initial step toward a study of the regulation of expression of specific genes at the molecular level, we also propose to isolate the mRNA coding for TP2. In order to do this we will need a cell free protein synthesis system for determining the biological activity of the mRNA. The purified mRNA will be used for the synthesis of a complementary copy of DNA (cDNA). This cDNA can then be used in DNA-RNA hybridization reactions to determine which spermatogenic cell types and which subcellular compartments (nuclei, polysomes, or postribosomal supernatant) in the various cell types contain the mRNA. For many of these experiments we will need relatively homogeneous populations of testicular cells and nuclei. Therefore we will use the techniques of velocity sedimentation, equilibrium density centrifugation and combinations of these techniques or other cell separation methods for these preparations.